A C8 aromatic compound is a very important raw material in the petrochemical industry. Examples of C8 aromatic compounds include xylenes, such as meta-xylene, paraxylene, ortho-xylene, and ethylbenzene. In particular, para-xylene is used as a main raw material for polyester fiber, and is separated from a C8 aromatic compound, which is commercially produced by reforming and cracking naphtha.
General methods of separating paraxylene from a C8 aromatic compound include the steps of separating paraxylene from the C8 aromatic compound through a unit process for the crystallization, adsorption and separation of the C8 aromatic compound, converting raffinate, which is the residual C8 aromatic compound, into a paraxylene-containing C8 aromatic compound by introducing the raffinate into an isomerization unit for forming paraxylene, and additionally separating and collecting paraxylene from the paraxylene-containing C8 aromatic compound by refluxing the paraxylene-containing C8 aromatic compound to a separation unit.
Generally, a C8 aromatic compound, which is produced by reforming and cracking naphtha, includes ethylbenzene, having a benzene ring bonded with one ethyl group, and xylene, having a benzene ring bonded with two methyl groups. When the C8 aromatic compound is isomerized, a disproportionation reaction, in which one methyl group of xylene is bonded to another xylene, thus producing one toluene and one trimethylbenzene, occurs. Therefore, the disproportion reaction incurs the loss of xylene during the isomerization process. 1% xylene loss has a cumulative economic effect of several billions to several tens of billions of/year on petrochemical companies, depending on the scale thereof.
In a process for producing paraxylene or ortho-xylene, isomerization catalysts generally serve to facilitate the isomerization reaction between xylene isomers, and serve to convert ethylbenzene into benzene through a dealkylation reaction, or convert the ethylbenzene into a xylene isomer through the isomerization reaction. A zeolite catalyst supported with a metal component of the VIII group of the periodic table is currently used as a commercial isomerization catalyst.
Examples of conventional zeolite catalysts supported with various metal components and used as catalysts for isomerizing xylene and converting ethylbenzene are as follows.
U.S. Pat. No. 4,939,110 discloses a method of producing a catalyst for isomerizing a C8 aromatic compound, comprising the steps of mixing an inorganic oxide binder, such as gamma-aluminum, with a pentasil zeolite, such as ZSM-5, such that the amount of the zeolite is 1˜20 wt %, to form a carrier, and supporting the carrier with 0.01˜2 wt % of a VIII group metal, such as platinum, and lead such that the amount of lead is 2˜10 times the amount of the VIII group metal. In the method, 80˜100% of the VIII group metal and 60˜100% of the lead are supported on the inorganic oxide binder, rather than the zeolite. As the result of isomerizing a C8 aromatic compound containing about 10 wt % of ethylbenzene on the catalyst, it was found that xylene loss is 0.8˜1.5 wt % when ethylbenzene conversion is 65 wt %. However, the method is problematic in that xylene loss is large compared to ethylbenzene conversion, and the lead content is very high.
Furthermore, U.S. Pat. No. 4,482,773 (Examples 1 to 3) discloses a method of a catalyst for isomerizing xylene and converting ethylbenzene, in which the catalyst is produced by impregnating ZSM-5, including no binder, with platinum and magnesium. In the method, in the case of the catalyst produced by impregnating the ZSM-5 into 2.4 wt % of a magnesium nitrate solution and then impregnating the ZSM-5 with 0.1 wt % of platinum, xylene loss is about 2.7 wt % when ethylbenzene conversion is 70 wt %.
The above conventional methods are problematic in that the loss of xylene must occur at a constant level in order to obtain a constant conversion rate of ethylbenzene.